1. Field of the Invention
This invention generally relates to the field of optical fibers, and more particularly to a refractive index profile of a phosphorus and rare earth doped fiber.
2. Description of the Related Art
Rare earth doped fiber, such as Ytterbium (Yb) or Erbium (Er) doped fiber is widely used as gain media for optical amplifiers. For example, most communication wavelength bands are determined by the Er gain wavelength. With bandwidth demand increasing exponentially, using the entire Er band has been viewed as an urgent and cost effective approach to meet the high demand for bandwidth.
Phosphorus-doped Er (Pxe2x80x94Si Er) fiber has been demonstrated to extend the amplification of the fiber to 1620 nm. This is shown, for example in the article entitled xe2x80x9cOptical Amplification Characteristics around 1.58 xcexcm of Silica-Based Erbium-Doped Fibers Containing Phosphorous/Alumina as Codopantsxe2x80x9d, by Kakui et al. The five valence phospotus P ion creates a local crystal field which interacts with Er F-shell electrons and reduces the Er excited state absorption probability and in turn extends the gain to longer wavelengths, such as 1620 nm. In the Pxe2x80x94Si Er fiber, P2O5 not only contributes to the gain in the long wavelength, but it also gives the fiber necessary waveguide properties, by affecting the refractive index of the core. However, P2O5 has a high vapor pressure, which imposes two limitations on the Pxe2x80x94Si profile: a low index ( less than 1%) and centerline burnout.
Pxe2x80x94Si Er fibers fabricated using standard Modified Chemical Vapor Deposition (MCVD) processes are well known in the art. In this process, the dopants, in gaseous form, are caused to flow into one end of a silica tube. The tube is then heated to ignite the chemicals and cause a reaction forming small glass particles known as xe2x80x9csoot,xe2x80x9d which is deposited onto the inner surface of the tube. The tube is heated to sinter the particles to form layers of glass corresponding to the core and the cladding of the fiber. This process of soot deposition and sintering is repeated until the desired glass material is formed. The tube is then collapsed under reduced heat conditions to form a glass rod from which an optical fiber can be drawn.
Unfortunately, there is extensive diffusion of phosphorus into the clad layers and significant evaporation of phosphorus from the inner surface of the tube during the collapse stage of MCVD. The result is a sizeable dip (xe2x80x9ccenterline dipxe2x80x9d) in the concentration of phosphorus along the center of the core. With a lower phosphorus concentration in the core center, there is a considerable dip in the refractive index along the centerline of the core. This centerline dip can reduce mode confinement in the core and thus decrease the overlap between Er ions and optical pump power.
One method developed in an attempt to solve this problem is a partial collapse step in the MCVD process. See R. A. Betts et al., xe2x80x9cOptical Amplifiers Based On Phosphorous Co-doped Rare-earth-doped Optical Fibresxe2x80x9d, International Journal of Optoelectronics, Vol. 6, Nos. 1/2, pp. 47-64. However, this is only a partial solution resulting in a fiber with only a slightly less drastic centerline dip. Accordingly, it would be desirable to provide a fiber structure which can compensate for the centerline dip in the refractive index for phosphorus- and rare earth-doped fibers used for optical amplification.
These and other drawbacks and limitations of conventional phosphorus- and rare earth-doped fiber for optical amplification are overcome according to exemplary embodiments wherein a fiber with a depressed inner cladding is provided. Instead of removing the centerline dip, the depressed inner cladding compensates for the centerline dip.
According to one embodiment of the present invention, an optical fiber for signal amplification comprises a core, doped with at least 15% phosphorus by weight and at least one rare earth ion sufficient to provide amplification of an optical signal, having a center axis. The optical fiber also comprises a clad layer, surrounding the core, having an axis collinear with the center axis of the core. The clad layer has an inner portion adjacent to the outer periphery of the core, and an outer portion. The inner portion can have an inner radius r1, preferably in the range of 2 xcexcm to 6 xcexcm, and an outer radius r2, the difference between which is preferably in the range of 2r1 to 5r1. The refractive index of the inner portion is less than the refractive index at the outer periphery of the core, so that the difference in the refractive index at the outer periphery of the core and the clad layer is sufficient to confine electromagnetic field within a selected wavelength range substantially only to the core. Preferably, the outer portion has a refractive index that is larger than the refractive index of the inner portion. Also preferably, the phosphorus dopant in the core is at least 20% by weight.
In a preferred aspect of this embodiment, the refractive index of the inner portion of the clad layer is in the range of 1.430 to less than 1.444 at 1550 nm. More preferably, the refractive index of the inner portion is between 1.440 and 1.442. Most preferably, the refractive index of the inner portion is about 1.441.
In another preferred aspect of this embodiment, the at least one rare earth ion includes erbium and preferably the optical fiber contains 0.2 to 0.4% of Er2O3 in the core by weight. The at least one rare earth ion can also include ytterbium. Also preferably the refractive index of the core is lowest along the center axis and initially increases with radial distance from the center axis. Also preferably, the concentration of phosphorus in the core is lowest along the center axis and initially increases with radial distance from the center axis.
In another preferred aspect of this embodiment, the selected wavelength range can be about 1560 nm to about 1640 nm.
According to another embodiment of the present invention, an optical fiber comprises a rare earth ion doped core having a center axis, where the refractive index is lowest along the center axis and initially increases with radial distance from the center axis. The optical fiber also comprises a clad layer having an axis collinear with the center axis having an inner portion adjacent to an outer periphery of the core and having a refractive index less than the refractive index at the outer periphery of the core, whereby the difference in refractive index between the outer periphery of the core and the clad layer is sufficient to confine electromagnetic field within a selected wavelength substantially only to the core. The clad layer also comprises at least one outer portion. Preferably, the outer portion of the clad layer has a larger index of refraction than does the inner portion of the clad layer. The optical fiber also has a mode field diameter of less than 7 xcexcm at the wavelength of 1550 nm. Preferably the selected wavelength range is 1560 nm to 1640 nm.
According to another embodiment of the present invention, an optical fiber comprises a core doped with phosphorus and erbium to a level sufficient to provide optical amplification with a center axis, where the refractive index of the core is lowest along the center axis and initially increases with radial distance from the center axis, the concentration of phosphorus in the core initially increases with radial distance from the center axis; the core has an outer radius in the range of 2 xcexcm to 6 xcexcm. The optical fiber also comprises a clad layer with an axis collinear with the center axis, an inner portion adjacent to an outer periphery of the core, and at least one outer portion, where the refractive index of the inner portion of the clad layer is less than the refractive index at the outer periphery of the core and the inner portion of the clad layer is sufficient to confine electromagnetic field within a wavelength range of 1560 nm to 1640 nm to substantially only the core; the inner portion has an inner radius and an outer radius and the difference r2xe2x88x92r1 is in the range of 2r1 to 5r1; and the clad layer includes at least one dopant selected from the group consisting of boron and fluorine. Preferably the optical fiber has an absorption of at least 20 dB/meter. Also preferably, the optical fiber has a mode field diameter of not more than 7 xcexcm at the wavelength of 1550 nm.
Other advantages and innovations of the present invention will become apparent to those skilled in the art from the detailed description which follows and the accompanying figures. Furthermore, the preceding summary and the following description are illustrative only and do not restrict the present invention as claimed.